Resistors are commonly used in integrated circuits such as silicon integrated circuits. For application in analog or in mixed mode analog and digital circuits, matching properties of resistors are very important.
Two types of resistors are generally used in integrated circuits: polysilicon resistors, and diffused resistors. In a conventional single-polysilicon CMOS process, to obtain good matching properties both types of resistors occupy large silicon area, for the following reasons.
The polysilicon resistor has good matching properties for the reason that its width is determined by a width of the silicon gate, which is a very tightly controlled dimension in a semiconductor manufacturing process. However, in a single-polysilicon complementary metal oxide silicon (CMOS) process the polysilicon has a very low sheet resistivity. As a result it requires large silicon area in practical implementations. In addition it has large parasitic capacitance (of the gate oxide) to silicon substrate, limiting its usage in very high speed applications.
The diffused resistor has higher sheet resistivity than polysilicon. As a result, for a given resistance value its length to width ratio is smaller than for a polysilicon resistor. However, matching properties of the diffused resistor are not good. This is caused by the resistor width being affected by its transition from its active region to the surrounding isolation region (referred to by persons skilled in the art as the LOCOS bird's beak). For very good matching the resistor width must be significantly larger than a minimum diffusion width. As a result a region occupied by the resistor is large. A large region causes a parasitic junction capacitance to be large, and as well limits resistor usage in very high speed applications.
To avoid the above described problems while achieving good resistor matching properties, a double-polysilicon CMOS process can be used. In this process a second polysilicon layer can have a larger sheet resistivity than polysilicon in a single polysilicon process. As a result some of the region and matching problems can be alleviated. However the double-polysilicon process is more complex and expensive than the single polysilicon process.